Wall of Serpents is also published as The Enchanter Compleated (1980).
THE ENCHANTER COMPLEATED!
His name was Harold Shea and he lived, quite simply, in Ohio, though he'd found his wife, Belphebe, in an Elizabethan wood, and lost two friends to the lures of Xanadu. And unless one of them, Pete the cop, showed up soon – back in Ohio, Harold would to under arrest for kidnapping an officer of the law.
So Harold, amateur enchanter, and Belphebe magicked off to find Pete – only to find themselves in the land of the Finnish Kalevala, facing a horde of angry warriors and a seething wall of serpents.
When a local sorcerer conjured up dozens of Harolds and Belphebes, it looked as if they had a pretty good chance of escaping in the general confusion. Then all the duplicates suddenly decided to disappear...